Transmitters are typically designed to transmit output signal within allocated channels. A transmitted output signal extending beyond its allocated channel may cause interference with an adjacent channel. This is known as adjacent channel emission.
For a phase modulated (PM) transmitter, for example, rapid phase transitions of a phase modulated waveform can create a wide bandwidth signal that is then forced through low-pass analog filters to provide rejection of the sampling artifacts. The low-pass filter bandwidth is much narrower than the bandwidth of the original signal. This may result in a substantial amplitude component on the filtered phase modulated signal.
The filtered phase modulated signal is then typically applied to a transmit gain controller. The transmit gain controller operates based on a transmit gain control (TGC) loop that applies a fixed DC value as a reference envelope control signal to reduce envelope variations in the filtered phase modulated signal. However, the TGC loop has a wider bandwidth than the filter bandwidth allowing the high frequency components to be re-introduced. This leads to high frequency components being introduced while “flattening” the envelope of the filtered phase modulated signal during phase transitions.
The remaining amplitude components and introduction of high frequency components result in extra energy in the adjacent channels, i.e., adjacent channel emission. In other words, the TGC loop is creating spectral regrowth by introducing the high frequency components while attempting to “flatten” the output transmit signal during phase transitions.
One approach for reducing spectral regrowth is to modify the transmitter by adding a filter to the output of the TGC loop. While effective in terms of performance, this approach may not be cost effective for existing transmitters already fielded since hardware modifications are needed.
Yet another approach for reducing spectral regrowth is disclosed in U.S. Patent Application No. 2004/0017859, A transmitter includes a pre-distorter to improve linearity of a power amplifier providing an amplified transmission signal. The amplified transmission signal is conditioned into a narrowband feedback signal that is responsive to a logarithm of the power appearing in the out-of-band components of the amplified transmission signal. The feedback signal is processed in a pre-distortion processor that implements an algorithm to adapt pre-distortion functions implemented in the pre-distorter, which is to improve linearity over time. The algorithm tests a population of randomly-generated pre-distortion functions. A baseline component of the coefficients from pre-distortion functions used in a subsequent population tracks the best-fit pre-distortion function from the current population. New populations are generated from old populations. While this approach is effective in terms of performance, it also is not cost effective for existing transmitters already fielded since hardware modifications are needed.